Laminates to be used in the substrates of printed circuits and HDD suspensions have been manufactured by pasting polyimide or polyester films to metal foils by means of an adhesive based on epoxy or acrylic rein. However, laminates manufactured in this manner show poor heat resistance because of the presence of an adhesive layer and, when subjected to a heat treatment, undergo dimensional changes to such an extent as to hinder the following steps. To solve this problem, laminates composed of metal foils and polyimide resins exhibiting good adhesiveness to metal foils and high heat resistance have been devised as disclosed in JP 5-22399 B. However, laminates composed of those polyimide resins which possess the aforementioned properties are generally processed by dry etching by the ultraviolet laser or plasma process and this practice has causes problems such as costly equipment, high running cost including the cost of gases to be used and poor quality for mass production.
Under the circumstances, materials processible by wet etching with organic alkalis or aqueous solutions of inorganic alkalis as etchants have been in demand as substitutes for materials processible by the aforementioned dry etching. However; the use of organic alkalis such as hydrazine presents a problem of high toxicity. Typical materials processible by wet etching with an aqueous alkaline solution are polyimide resins Kapton of duPont and Apical of Kaneka Corporation and the processing technique proposed for these polyimide resins consists of directly plating the resins with metals and etching the plated resins in an arbitrary shape. This technique, however, faces problems such as an increased number of processing steps and poor dimensional accuracy and is further restricted by the characteristics of metals in use. Another proposal made in JP6-322126 A involves direct lamination of a polyimide resin of low thermal expansion to a metal foil to provide a material processible by wet etching with an aqueous alkaline solution; however, no interest at all is expressed here in the subject of adhesiveness and the material in question is restricted in use, for example, a laminate having metal foils on both sides cannot be made from the polyimide resin in question because of its characteristically high glass transition temperature (Tg). Still further, lamination is effected by directly applying a polyimide resin to a metal foil followed by drying according to the aforementioned process and, although a laminate of polyimide resin and metal foil shows good planarity, the polyimide layer by itself develops warpage. This leads to a problematical situation where warpage and undulation develop readily during etching of the metal foil. Therefore, there has been a strong demand for laminates composed of metal foils and polyimide resins that are processible by wet etching and exhibit high heat resistance and good adhesiveness to metal foils.